The color temperature of a light, measured in kelvins, is its most noticeable characteristic. A candle, at 1900 K, appears orange. Daylight, at 5500 K, is much whiter and bluer. A 5500-K bulb, however, will not necessarily produce the same light as the sun; for the full nature of a light source, one must look to its spectral power distribution (SPD) curve, which describes its irradiance across the entire visible spectrum. An incandescent bulb achieves its temperature by emitting light over a smooth curve, with the balance tilted toward yellow and red. A CFL, and to a lesser extent an LED, mimics incandescents using a different mixture of light, with spikes and troughs of power strategically positioned across the spectrum to create a correlated—or averaged—color temperature. This results in less faithful reproduction of colors, expressed as a diminished color rendering index (CRI).How We TestedReadings for color temperature, color rendering and spectral distribution were taken with an Ocean Optics USB2000 Miniature Fiber Optic Spectrometer, provided and calibrated by SpectrEcology of Jasper, Ga. Subjective testing was performed by volunteers in PM's test lab who observed bulbs hidden behind a white shade.

Decoding the Science of Light

Decoding the Science of Light

The color temperature of a light, measured in kelvins, is its most noticeable characteristic. A candle, at 1900 K, appears orange. Daylight, at 5500 K, is much whiter and bluer. A 5500-K bulb, however, will not necessarily produce the same light as the sun; for the full nature of a light source, one must look to its spectral power distribution (SPD) curve, which describes its irradiance across the entire visible spectrum. An incandescent bulb achieves its temperature by emitting light over a smooth curve, with the balance tilted toward yellow and red. A CFL, and to a lesser extent an LED, mimics incandescents using a different mixture of light, with spikes and troughs of power strategically positioned across the spectrum to create a correlated—or averaged—color temperature. This results in less faithful reproduction of colors, expressed as a diminished color rendering index (CRI).

How We Tested

Readings for color temperature, color rendering and spectral distribution were taken with an Ocean Optics USB2000 Miniature Fiber Optic Spectrometer, provided and calibrated by SpectrEcology of Jasper, Ga. Subjective testing was performed by volunteers in PM's test lab who observed bulbs hidden behind a white shade.

Incandescent Bulbs

Incandescent Bulbs

An incandescent bulb emits light by heating a tungsten filament surrounded by various inert gases to about 4000 F. These bulbs light well and cost little, but die quickly and are highly inefficient, releasing 90 percent of their energy as heat. In incandescent bulbs, PM's testers noted yellow hues and warm light.